In the field of infrared detectors, the use of devices configured in the form of an array and capable of operating at ambient temperature, i.e. not requiring cooling to extremely low temperatures, is known—in contrast to detecting devices referred to as “quantic detectors” which can only operate at extremely low temperatures, typically that of liquid nitrogen.
These uncooled detectors traditionally use the variation in a physical unit of an appropriate material as a function of temperature at around 300 K. In the case of bolometric detectors, this physical unit is electrical resistivity.
Such an uncooled detector generally includes:                means of absorbing the infrared radiation and converting it into heat,        means of thermally isolating the detector so that its temperature can rise due to the effect of the infrared radiation,        thermometric means which, in the context of a bolometric detector, use a resistance element made up of electrodes and a sensitive material which is referred to as a bolometric material,        and means of reading electrical signals provided by the thermometric means.        
The means of absorbing the radiation and the thermometric means are integrated in a membrane which is suspended, by thermal isolation means, above a substrate in which the means of reading are provided.
Detectors intended for infrared imaging are conventionally produced as a one- or two-dimensional array of elementary detectors, said array being “monolithic” or mounted on a substrate generally made of silicon which incorporates means of sequentially addressing the elementary detectors and means of electrical excitation and of pre-processing the electrical signals generated by these elementary detectors. This substrate and the integrated means are commonly denoted by the term “readout circuit”.
In order to obtain an image using this detector, the radiation obtained from the scene is projected through suitable optics onto the array of elementary detectors and clocked electrical stimuli are applied via the readout circuit to each of the elementary detectors or to each row of such detectors in order to obtain an electrical signal that constitutes an image of the temperature reached by each of said elementary detectors. This signal is then processed to a greater or lesser extent by the readout circuit and then, if applicable, by an electronic device outside the package in order to generate a thermal image of the observed scene.
The essential difficulty of using bolometric detectors is the extremely small relative variation in their electrical resistivity that is representative of the local temperature variations of an observed scene compared with the average value of these resistances.
In fact, the physical laws of thermal emission in the infrared spectrum of the observed scene typically from 8 to 14 micrometers (equivalent to the transparency band of the terrestrial atmosphere in which bolometric detectors are usually used) result in a differential power dP on the detector's focal plane of the order of 50 μW/cm2 when the temperature of the scene varies 1 K either side of 300 K. Determining this value is easily within the capabilities of those skilled in the art by applying the above-mentioned physical laws.
This estimate is valid for an f/1 optics, good transmission between the scene and detector and if the detector only receives a negligible amount of energy outside the specified wavelength band, for example and typically, if the package has a window that is transparent in this range and opaque below and beyond the stated limits.
Consequently, the variation in temperature dθ of a bolometer working at thermal equilibrium associated with an infrared power dP absorbed on its surface S is given by the following equation:dT=Rth·dP   (1)where Rth is the thermal resistance between the sensitive part of the bolometer, the temperature of which rises due to the infrared radiation, and the isothermal substrate on which it is mounted.
Thus, for a bolometer of typical dimensions of the order of 30 μm×30 μm which represents a surface area of 9.10−6 cm2, the typical thermal resistance according to the prior art is of the order of 20 to 60 MK/W which results in an increase in the temperature of the bolometer of the order of 0.01 K to 0.03 K if the temperature of the element of the scene observed by the bolometer varies by 1 K.
If Rb is the electrical resistance across the two current input terminals on the sensitive bolometric material, the resulting variation in resistance dRb is expressed by the following equation:dRb=Rb·TCR·dθ  (2)where TCR is the relative coefficient of variation in resistance of the material that constitutes the sensitive part of the bolometer at around its operating temperature θ. For the usual materials in this field (vanadium oxides, amorphous silicon), this coefficient TCR is approximately −2% per K. Consequently, the relative variation in resistance dR/R resulting from a difference of 1 K over the scene is therefore of the order of 0.04%, i.e. 4.10−4/K.
Nowadays, thermal imaging resolutions much better than 1 K, typically 0.05 K or even less are required. Such results can be obtained by producing structures that have very high thermal resistances Rth by using sophisticated techniques. However, there remains the need to measure minute relative variations in resistance, typically—as stated earlier—of the order of 10−6 in order to resolve temperature variations in time and space of just a few dozen millikelvins.
In order to explain the difficulty of analyzing such a small variation, FIG. 1 shows a schematic view of a readout circuit. A predetermined constant bias voltage VDDA is applied to one of the terminals of a resistive bolometer 12 which has a resistance Rb and is exposed to infrared radiation. The readout circuit comprises an integrator 10 which comprises:                an operational amplifier 14, the non-inverting input (+) of which is kept at a predetermined constant voltage VBUS;        a capacitor 16, having a predetermined capacitance Cint and connected between the inverting input (−) of amplifier 14 and the output of the latter;        a reset switch 18 connected in parallel with capacitor 16 and controllable by means of a “Reset” signal.        
The readout circuit also comprises:                a first read switch 20, controllable by means of a “select” signal and connected to the inverting input (−) of operational amplifier 14;        a MOS injection transistor 22, the gate of which is kept at a constant predetermined voltage GFID, the source of which is connected to the other terminal of bolometer 12 and the drain of which is connected to the other terminal of first read switch 20; and        a data processing unit 23 which is connected to the output of operational amplifier 14 and determines the variation in the resistance of bolometer 12 caused by the infrared radiation received by the latter, and hence this infrared radiation, as a function of the voltage Vout on the output of the operational amplifier.        
At the start of a cycle to read bolometer 12, reset switch 18, which is closed following a cycle to discharge capacitor 16, is switched to the open state by adjusting the “reset” signal to an appropriate value. The first read switch 20 which is in the open state is switched to the closed state by adjusting the “select” signal. The current which flows through bolometer 12 is then integrated by capacitor 16. When a predetermined integration period ΔTint has elapsed since the start of the readout cycle, the first read switch 20 is switched to its open state. The voltage Vout on the output of this integrator, an image of the resistance of Rb of the bolometer, is given by the equation:
                              V          out                =                                            V              pol                                      R              b                                ×                                    Δ              ⁢                                                          ⁢                              T                int                                                    C              int                                                          (        3        )            where Vpol is the bias voltage across the terminals of bolometer 12, assuming, for the sake of simplicity, that Rb varies little throughout integration period Tint.
Thus, an array of N resistors (bolometers) can be read using this principle with the aid of simultaneous integration (by means of N integrators) or sequential integration (in an integrator at the end of a row or end of a column or even a single integrator for the array).
If the array thus produced is illuminated by projecting an infrared scene, Vout will provide variations in space (obtained from each bolometer) representative of the scene. The reader is reminded that voltage Vout as stated previously consists largely of a component that is constant from one detector to the other and which therefore has no relevance in terms of imaging.
Also, because there is thermal coupling between the substrate and the bolometer, the thermal variations to which the substrate is subjected have an effect on the bolometer. Because conventional bolometers are extremely sensitive to such variations, this stray current component interferes with the output signal and this has an adverse effect on the quality of infrared radiation detection.
Collectively, these components, namely the constant part of Vout and the above-mentioned interference due to the substrate, which are contained in the signal Vout, are usually called the “common mode” signal.
Finally, reading a bolometer causes an electrical current to flow through it. The temperature of the bolometer therefore rises due to the Joule effect (this is usually called “self-heating”), thus adding more interference which disturbs the wanted signal associated with the scene. This self-heating Δθ(t) is a function of time and can be determined by using the following differential equation:
            C      th        ×                            ∂          Δ                ⁢                                  ⁢                  θ          ⁡                      (            t            )                                      ∂        t              =                    V        pol        2                              R          b                ⁡                  (                      θ            ⁡                          (              t              )                                )                      -                  Δ        ⁢                                  ⁢                  θ          ⁡                      (            t            )                                      R        th            where Cth is the heat capacity of the sensitive membrane.
For a very short integration time Tint of the order of a few dozen microseconds after applying voltage Vpol at t=0, this increase in temperature can be considered to be linear and is given by the equation:
      Δ    ⁢                  ⁢    θ    =                    V        pol        2                              C          th                ×                              R            b                    ⁡                      (                          θ              ⁡                              (                                  t                  =                  0                                )                                      )                                ⁢          T      int      
It would appear that, for typical values of Cth, Rth, Vpol and Tint, this increase in temperature due to self-heating is typically as much as several degrees Kelvin. Thus, even though they are very limited, e.g. roughly 1%, variations in Cth, or Rb in space which are inherent in the technology result in spatial temperature variations of each membrane at the end of an integration time of approximately 20 mK for an electrical temperature increase Δθ of 2°, i.e. of the same order as the temperature increase caused by a 1 K increase in the scene temperature.
These variations also interfere with the representativeness of signal Vout compared with the variations in radiant flux in space and in time which are alone representative of the observed scene and constitute the wanted signal.
A so-called “reference” resistive structure described in the document entitled “Performance of 320×240 Uncooled Bolometer-type Infrared Focal Plane Arrays” by Yutaka Tanake et al., Proc. SPIE, vol 5074 has been proposed in order to overcome these drawbacks.
The principle of such a reference resistive structure is to associate a second identical resistive bolometer, which is biased and connected identically to the first bolometer, with the resistive bolometer 12 in FIG. 1. The second bolometer is also designed to be essentially insensitive to the flux obtained from the scene, typically by providing an opaque metallic membrane. The first and second resistive bolometers are also associated so that the current that flows through the second bolometer is subtracted from the current that flows through the first bolometer and it is this differential current that is used by the readout circuit.
In order to distinguish the functions of these two bolometers, the term “imaging bolometer” will be used to denote the first bolometer and the term “reference bolometer” will be used to denote the second bolometer even though, in certain applications such as thermometry for example, an image is not necessarily produced and a temperature measurement, for instance, is made.
A reference structure 24 is schematically shown in FIG. 2; this includes the components as in FIG. 1 and “reference” circuit 24 is associated with these. Reference circuit 24 comprises reference bolometer 26, MOS bias transistor 28 and a second read switch 30 which are, respectively, substantially identical to imaging bolometer 12, MOS injection transistor 22 and first read switch 20.
Components 26, 28 and 30 are, moreover, biased and designed the same way as components 12, 22 and 20, apart from the fact that reference bolometer 26 has an opaque metallic membrane 32 which protects it against radiation originating from the scene.
Finally, the resistive reference structure comprises a current mirror 34, one input leg of which is connected to terminal A of second read switch 30 and the other input leg of which is connected to terminal B of first read switch 20. This current mirror 34 substantially reproduces the current i1 that flows through reference bolometer 26 on terminal B.
Using current mirrors makes it possible to have only a single reference structure per row with all these structures being arranged in a reference “column” for a matrix detector. Current mirrors are structures that are familiar to those skilled in the art. Generally speaking, they make it possible to copy a reference current from a remote structure and, in particular, to distribute this reference current to a multitude of circuitry components, regardless of their resistive load.
Thus, the current i1 that flows through the reference bolometer is substantially equal to the common mode current and the reference bolometer and the imaging bolometer are both subjected to the same thermal variations originating from the substrate. The difference i2-i1 between current i2 that flows through the imaging bolometer and current i1 that flows through the reference bolometer is then substantially unaffected by interference consisting of the common mode current and the component associated with thermal variations of the substrate, at least provided one can assume that the substrate is essentially isothermal. This differential current i2-i1 therefore corresponds substantially to the current produced by the variation in the resistance of imaging bolometer 12 due to the increase in its temperature caused by infrared radiation obtained from the scene.
There are two classic layouts that use reference bolometers.
In a first layout shown in FIG. 3, a reference bolometer 26 is provided for every row in array 50 of imaging bolometers and therefore supplies, via current mirror 34, a so-called “reference” current for all the imaging bolometers in the row. The self-heating phenomena that affect imaging bolometers 12 are therefore compensated because reference bolometer 26 is subjected to the same bias cycles as imaging bolometers 12 in the associated row. On the other hand, providing a reference bolometer 26 for every row of the array of imaging bolometers generates spatial noise on that row because of the technological dispersions associated with reference bolometers.
In a second layout shown in FIG. 4, just a single reference bolometer 26 is provided for all the imaging bolometers in array 50. The current output by the latter is then copied by a set of current mirrors 34. This avoids the spatial noise produced by technological dispersions. Nevertheless, the thermal cycle of this single reference bolometer 26 is substantially different to that of imaging bolometers 12. In fact, unlike an imaging bolometer which is biased when its row is read, reference bolometer 26 is biased every time a row is read. The thermal time constant of the reference bolometer Rth.Cth of around several milliseconds does not allow it to return to its equilibrium temperature before each read (integration) cycle starts. Consequently, the reference bolometer starts the next integration cycle with a different initial resistance value. As a result, after a certain number of read cycles, self-heating of reference bolometer 26 differs substantially from the self-heating of imaging bolometers 12 so that rejection of this component is very mediocre.
Note that self-heating phenomena become more marked the more effectively the membranes of imaging bolometers 12 are isolated; this is the case in top of the range detectors where attempts are made to maximize Rth in order to maximize sensitivity, and this also leads to Cth being reduced in order to maintain a thermal time constant which is compatible with imaging frame frequency standards. Because of this, the temperature increase after each read operation becomes greater for comparable stimuli and ultimately the residual increase in temperature at the start of the next cycle is even more critical.
Common mode current rejection therefore becomes more awkward the more sensitive the detector is and, ultimately, this limits the performance improvements which can be made to these detectors.
Besides this, a resistive reference structure is technically difficult to produce. In fact, to obtain satisfactory operation of such a structure, metallic membrane 26 which protects the reference bolometer must be totally impermeable to the flux originating from the scene and it must be thermally isolated from the other elements of the structure in order to prevent the reference bolometer being affected by any thermal interference. Such a membrane is difficult to design and produce. In addition, the significant complexity introduced by fabricating such a membrane necessarily involves additional cost due to additional fabrication processes and the fact that production yields are not ideal.